Industrially water and ethanol often are present in combinations which are not optimal for further use of any one of these compounds. Ethanol as a raw material is suitable as a solvent for a plurality of processes or as an energy source. As an example combinations of water and ethanol are present in varying ratios in processes which have as an object to provide substantially water free ethanol as an energy source produced from biological raw materials. In this connection the ethanol is typically referred to as bioethanol.
Traditional distillation process is typically used for the manufacture of ethanol with a water content somewhat higher than the azeotropic point of the composition. In order to reduce the water content of the ethanol/water composition further, to thereby enhance the utility of the ethanol and its value as a product, different methods have been attempted as described in the following paragraphs.
Multi-component distillation (also referred to as azeotropic distillation) has been, and still is, used for this purpose. A comparatively high energy consumption and use of chemicals are characteristic features of this technique. There are known process solutions for multi-component distillation in which energy recovery, hereby included mechanical recompression of process steam, is included. Examples are found in JP patent 59196833, FR patent 2855170 and JP patent 60226837, which describe distillation columns operated in cascade in combination with recompression of process steam.
Another method is the so-called extractive distillation. Also this method involves the use of chemicals for the extraction step. U.S. Pat. No. 5,294,304 and JP patent 61254177 describes such processes in which recompression of process steam is included in the total energy system.
Two other techniques are molecular sieve and vapour permeation in which upstream vapour is generated in evaporator or taken directly from a traditional distillation process. Discharge flows in such techniques are a retentate (substantially water-free ethanol) and a permeate containing the water with a larger or smaller portion of ethanol which is typically condensed out and then recovered in distillation column. Traditional processes with evaporator consume energy for the evaporation of the feed liquid and for the recovery of the permeate ethanol portion, inclusive the generation of reflux liquid in the distillation step of said permeate.
US patent 2007000769 describes traditional molecular sieve process without recompression of process vapour. The main difference between molecular sieve and vapour permeation is that with the molecular sieve the dewatering step as such is conducted batch wise, why typically a number of molecular sieve tanks are typically mutually interchanged and the tanks which are not in production are subject to a recovery process. In this manner the process upstream and downstream is operated in a continuous manner and the discharge flows from the dewatering unit of the molecular sieve is a retentate and a permeate like with the vapour permeation.
Membrane separation with the feed in liquid form through the membrane, so-called pervaporation, is also a technique that is used. The discharge permeate flow is also with this technique in form of vapour while the retentate is in liquid phase. The energy consumption is comparatively low in comparison with traditional vapour permeation since there is no need for energy supply to evaporate the retentate portion of the feed. The energy supplied is in stead used to evaporate the permeate and to recover the permeate ethanol portion, also included generation of reflux liquid to the distillation step of said permeate. Pervaporation is a less robust technique with respect to the lifetime of membranes, since the membranes are exposed to possible impurities in the feed. There are known processes involving pervaporation in which mechanical recompression of process vapour is included. As an example JP patent 63059308 describes a process for pervaporation of ethanol. The permeate from the membrane flows via a vapour compression step before being fed to the distillation step as feed vapour. Compression of process vapour in this process has the consequence that the permeate can be maintained as vapour into the distillation column, thereby reducing the energy amount which would else be needed for recovery of ethanol from the permeate.
Another example of recompression of permeate is described in JP patent 5137969, where a pressure increase of the permeate in vapour form is used to condense this at the available cooling water temperature in stead of having to use a cooling medium produced by an energy demanding refrigeration unit.